Can one LU be in multiple devices?

Mallikarjun C. cbm at rose.hp.com
Tue Sep 28 14:43:27 PDT 2004


* From the T10 Reflector (t10 at t10.org), posted by:
* "Mallikarjun C." <cbm at rose.hp.com>
*
Hi George,

Thanks for the response.

Some of us think (myself included) that it is desireable in future to 
explicitly allow an LU to be visible through multiple SCSI devices. 
Eventhough SAM-3 disallows it as you confirmed, SAM-4 could allow it in 
the new UML drawings.  I do not see doing so affecting the currently 
defined composition of an LU (device server, WWN etc.), but I may not be 
seeing the big picture.  I would like to know more about the issues you 
see in allowing multi-device visibility of an LU in SAM-4.

Regards.
-- 
Mallikarjun

Mallikarjun Chadalapaka
Networked Storage Architecture
Network Storage Solutions
Hewlett-Packard MS 5668
Roseville CA 95747
cbm [at] rose.hp.com




George Penokie wrote:

> 
> Mallikarjun,
> 
> A logical unit can only exist in a single target device and each logical 
> unit has it's own device server.  This is intentional and should become 
> more clear as the UML drawings are developed for SAM-4 (look for 
> 04-023r2 to appear within the next few weeks).
> 
> A logical unit may be accessed through multiple target ports by multiple 
> application clients. Or, if you prefer, a single logical unit can be 
> accessed using any number of I_T nexuses.
> 
> Bye for now,
> George Penokie
> 
> Dept 2C6  114-2 N212
> E-Mail:    gop at us.ibm.com
> Internal:  553-5208
> External: 507-253-5208   FAX: 507-253-2880
> 
> 
> 
> 
> *"Mallikarjun C." <cbm at rose.hp.com>*
> Sent by: owner-t10 at t10.org
> 
> 09/27/2004 07:53 PM
> 
> 	
> To
> 	T10 Reflector <t10 at t10.org>
> cc
> 	Julian Satran <Julian_Satran at il.ibm.com>, "KRUEGER,MARJORIE 
> (HP-Roseville,ex1)" <marjorie.krueger at hp.com>, "Reuter, Jim" 
> <james.reuter at hp.com>
> Subject
> 	Can one LU be in multiple devices?
> 
> 
> 	
> 
> 
> 
> 
> 
> * From the T10 Reflector (t10 at t10.org), posted by:
> * "Mallikarjun C." <cbm at rose.hp.com>
> *
> iSCSI allows one "network entity" to host multiple SCSI devices. Since
> all the LU inventory is accesible in the same physical box, each LU can
> actually be a member of multiple SCSI devices and there are some
> advantages in doing that.  A similar situation exists for
> multi-networked devices and virtualization boxes.
> 
> I'd like to solicit input from this list on this question: is it
> SAM-3-compliant to have one LU appear in multiple devices?
> 
> Jim Hafner and I had an offline email discussion on this.  Jim's
> position was that LUs appearing in multiple devices is legal, because
> (paraphrasing him; Jim, please add/correct):
> - It is technically no different from dual-controller arrays (same LU
> can be presented on each I_T nexus, and the nexus ID doesn't include the
> device identifier).
> - Storage virtualizers do this today as well.
> - Target Reset task management function leads to different results if
> the LU is contained in two devices versus one device, but the recent
> obsolescence of Target Reset helps.
> 
> I tended to concur with him, but after looking through the SAM-3
> drawings, I am not sure.  Here's my reasoning:
> 
> 1. SAM-3's hierarchy diagrams show containment relationships (3.6.1
> makes this clear).
> 
> 2. Figure 11 and Figure 14 (in 4.7.2 & 4.8 respectively) together imply
> to me that a device server is contained within a SCSI device.
> 
> 3. From the same figures, the containment relationship between the
> target device object & the device server object is 1-to-1.  IOW, Figure
> 11 does not show a Logical Unit being contained in more than one SCSI
> target device (more precisely, the SCSI target device box is not shaded).
> 
> Putting these things together, having one LU (i.e. with all the
> composition shown in Figure 14) show up as a contained object in
> multiple discrete SCSI target device instances seems disallowed.
> 
> Now the questions are:  Is my reading of the drawings correct?  If so,
> is this prohibition (that each LU belongs to one and only SCSI device)
> intended?
> 
> Comments would help.  Thanks.
> -- 
> Mallikarjun
> 
> Mallikarjun Chadalapaka
> Networked Storage Architecture
> Network Storage Solutions
> Hewlett-Packard MS 5668
> Roseville CA 95747
> cbm [at] rose.hp.com
> 
> 
> 
> 
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